Neuropeptide Y (NPY) receptors in the paraventricular nucleus of the hypothalamus (PVN) process information from NPY afferents relaying sensory, cardiovascular and endocrine information from the brainstem and hypothalamus. Any perturbation in this system, such as that perceived during stressful conditions, will alter the activity of the PVN. Y1 and Y5 receptor levels in the PVN are in part responsible for fine tuning these inputs and producing an efferent signal that will affect corticotropin releasing factor (CRF) secretion, cardiovascular and sympathetic tone. The long- range objective of this proposal is to examine the anatomical distribution and regulation of Y1 and Y5 receptor subtypes within the PVN as they relate to the regulation of stress hormone secretion. The hypothesis that Y1 and Y5 receptors are present on neuroendocrine neurons that regulate CRF and adrenocorticotropin (ACTH) secretion will be assessed. In addition, we will test the hypotheses that Y1 and Y5 receptor expressing neurons are activated during stressful conditions and that NPY regulates the expression of its own receptors. The following specific aims will be tested: 1. Determine whether NPY neurons projecting to the PVN and Y1 and Y5 receptor-containing neurons are activated during stress: 2. Determine the ultrastructural localization of NPY Y1 and Y5 receptors within the PVN: 3. Identify the phenotype and projections of hypothalamic Y1 and Y5 receptor immunoreactive neurons in the PVN: 4. Examine the regulation and processing of Y1 and Y5 receptors by elevated levels of NPY. This will be accomplished by using a combination of anatomical tracing methods combined with single and double label immunocytochemistry at the light and electron microscope levels to determine the projections and phenotype of Y1 and Y5 receptive neurons in the PVN. Different stressors (physical, psychological and nutritional) and chronic administration of NPY will be used to test the responsiveness and regulation of Y1 and Y5 receptors to these challenges. These studies will provide significant information on the distribution and neuroanatomy of Y1 and Y5 receptor expressing cells in the PVN and their potential role in integrating stress responses in the PVN. The identification of NPY receptors on neural pathways and hypothalamic neuropeptide neurons will provide important information regarding the regulation and integration of stress hormone secretion and food intake. Dysregulation of these systems may be implicated in the development of stress related illness such as depression and eating disorders.